In recent years, it is considered to install more low-power Base Stations (BSs) (e.g., femto BSs) to compensate for a macro BS in a wireless communication system. The femto BS is a home BS or a Small Office Home Office (SOHO) BS that is installed in a region other than a service area of the macro BS or that is installed in a region where service quality is low due to weak signal strength. The femto BS provides a User Equipment (UE) with a portable Internet service similarly to the macro BS. Characteristics of the femto BS include low power, small capacity, low price, and the like, which are optimized for a small indoor environment (e.g., home or SOHO). Therefore, the femto BS has to interoperate with the macro BS, and a legacy UE has to receive a service by switching between the femto BS and the macro BS. That is, it is necessary to achieve a handover between the macro BS and the femto BS according to a wireless environment so that the UE may receive an improved service.
A femto cell is classified into a cell accessible by all UEs and a cell accessible by only an allowed UE. The cell accessible by only the allowed UE is referred to as a Closed Subscriber Group (CSG) cell. In an environment where the CSG cell coexists with a macro cell, the UE has to automatically search for a CSG cell located nearby without the aid of a network.
A method of performing automatic search on a CSG cell by a UE will be described below. First, the UE uses a normal gap pattern of the UE to verify a gap duration in which communication is not performed, and scans a neighbor cell during the verified gap duration. Herein, the gap duration is a scanning duration for searching for the neighbor cell. The UE receives a reference symbol from the neighbor cell during the gap duration, and obtains a Physical Cell IDentifier (PCID) of the neighbor cell by using the received reference symbol. Thereafter, the UE reports the obtained PCID of the neighbor cell to a serving macro BS by using a measurement report message. In this case, the serving macro BS allocates a long gap to the UE by using a gap allocation message with respect to a neighbor CSG cell of which radio wave strength greater than or equal to a specific threshold, and allows the UE to be able to receive a System Information Block (SIB) of the neighbor CSG cell during the long gap duration. Herein, the SIB includes a General Cell Identifier (GCID), which is a unique cell IDentifier (ID) of the neighbor cell and a CSG ID, which is a unique CSG cell ID. Therefore, the UE may obtain the CSG ID of the neighbor CSG cell. The UE compares the obtained CSG ID with a CSG ID of a white list stored in a Universal Subscriber Identity Module (USIM) of the UE, and thus may recognize whether the CSG cell is accessible CSG cell. The white list is a list of CSG cells accessible by the UE. Thereafter, the UE reports an accessible CSG cell to the serving macro BS by using the measurement report message, and the serving macro BS determines whether the UE may be handed over to the CSG cell according to the reported accessible CSG cell.
The method of performing automatic search on the CSG cell by the UE has the following problems. First, the CSG cell is searched for even if the UE does not support a CSG function, and the UE supporting the CSG function continuously searches for the CSG cell even in a region where the CSG cell does not exist, which results in battery waste of the UE. When using an asynchronous system, an SIB broadcast time of a neighbor CSG cell cannot be known, and thus the UE is assigned with a long gap having a length equal to or longer than an SIB broadcast period so as to allow the UE to obtain a CSG ID of the neighbor CSG cell during a corresponding long gap duration. In this case, data communication with respect to the UE is disconnected during the long gap duration, thereby causing a problem of user throughput deterioration in the UE. Further, another problem arises in that a user has to receive an SIB even in an active state. Furthermore, according to a ‘best cell principle’ of the conventional 3rd Generation Partnership Project (3GPP) system, a problem arises in that, when a discovered cell is superior to a current serving cell in terms of signal strength, or the like, even if the current cell receives a service without errors, the signal strength, or the like, are measured and reported so as to attempt handover or cell reselection. If this principle is also applied to a femto cell environment, there is a need to provide a method of occasionally turning off a neighbor cell search function of the UE.